Perpendicular magnetic recording disk

ABSTRACT

A perpendicular magnetic recording disk is provided. The perpendicular magnetic recording disk includes an underlayer between a substrate and a perpendicular magnetic recording layer for inducing perpendicular orientation of the perpendicular magnetic recording layer, the perpendicular magnetic recording layer having a thickness in the range where the ratio of perpendicular coercivity Hc to maximum perpendicular coercivity Ho decreases with reduced thickness of the perpendicular magnetic recording layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnetic recording disks, and moreparticularly, to single-layer and pseudo double-layer perpendicularmagnetic recording disks with microsized domains.

2. Description of the Related Art

In longitudinal magnetic recording (LMR) applied to hard disk drives(HDDs), a major external data storage device of computers, the size of adata record domain in a magnetic disk has decreased with microstructureas the need for high-density data recording increases. However, thisdecrease in size makes the data record domains susceptible to removal bythermal energy generated by operation of the HDD which is more dominantthan magnetostatic energy from the data record domain. This is referredto as the super paramagnetic effect. To overcome the super paramagneticeffect, the LMR technique has been replaced by a perpendicular magneticrecording (PMR) technique for HDD applications. The PMR technique uses ahigher electrostatic energy and lower demagnetization energy compared tothe LMR technique, so it is advantageous in high-density data recording.The high-density PMR technique also has enabled detection of a microdata domain in combination with advances in the manufacture of highlysensitive read heads.

In the PMR technique suitable for high-density magnetic recording,perpendicular magnetic anisotropy energy is exerted to orient thedirection of magnetized domains perpendicular to the plane of a magneticdisk. Thus, head fields from a magnetic head should be induced to beperpendicular to the magnetic disk plane and thus parallel to themagnetized domains. To achieve this, a single-pole-type (SPT)perpendicular magnetic head is required. However, the SPT perpendicularmagnetic head also generates a demagnetization field stronger than theperpendicular field of the magnetic head, so the perpendicular magneticfield induced by the SPT head is insufficient for recording, thuslimiting use of the perpendicular magnetic recording technique in HDDapplications.

The recent advances in magnetic recording technologies have enabled PMRwith a ring-type magnetic head that has been used widely in LMP due toits ability to apply enhanced perpendicular magnetic fields forrecording. Based on the PMR performed using the ring-type magnetic head,a single-layer PMR disk with a perpendicular magnetic recording/playbacklayer has been developed.

The schematic structure of a single-layer PMR disk is shown in FIG. 1.The single-layer PMR disk includes an underlayer 12 for promoting theperpendicular orientation of a perpendicular magnetic recording layer 13formed over the underlayer 12, the perpendicular magnetic recordinglayer 13 having the perpendicular magnetic anisotropy energy to keep theperpendicular orientation of the data record domain, a protective layer14 for protecting the perpendicular magnetic recording layer 13 fromexternal impacts, and a lubricant layer 15.

The perpendicular magnetic recording layer 13 has the perpendicularmagnetic anisotropy energy with a magnetic easy axis orientedperpendicular to the plane of the perpendicular magnetic recording layer13 due to the underlayer 12. Therefore, perpendicular data recording canbe achieved by perpendicular magnetic field components from a ring-typehead. However, in the conventional single-layer PMR disk shown in FIG.1, the perpendicular magnetic recording layer 13 having theperpendicular magnetic anisotropy energy has also a largedemagnetization factor and thus strong demagnetization energy is inducedin a direction opposite to the magnetic moment of the perpendicularmagnetic recording layer 13, as expressed by formula (1) below:Ku _(eff) =Ku−2πNdMs ²  (1)where Ku_(eff) is the effective perpendicular magnetic anisotropyenergy, Ku is the perpendicular magnetic anisotropy energy, Nd is thedemagnetization factor, Ms is the saturation magnetization, and 2πNdMs²is the demagnetization energy.

Thus, the effective perpendicular magnetic anisotropy energy of theperpendicular magnetic recording layer 13 is abruptly decreased withunsatisfactory high-density recording properties, thereby limiting HDDapplications of the perpendicular magnetic recording technique.

To overcome the effective perpendicular magnetic anisotropy energyreduction occurring in such a single-layer PMR disk, a pseudodouble-layer PMR disk capable of reducing the demagnetization energy ofits perpendicular magnetic recording layer has been developed.

In the pseudo double-layer PMR disk, as shown in FIG. 2, an intermediatesoft magnetic layer 26 is deposited between a perpendicular orientationpromoting underlayer 22 and a perpendicular magnetic recording layer 23to allow formation of a closed magnetic circuit through theperpendicular magnetic recording layer 23 by perpendicular magneticfield components from a ring-type head. The closed magnetic circuitformed by the intermediate soft magnetic layer 26 reduces thedemagnetization factor of the perpendicular magnetic recording layer 23and its demagnetization energy, and thereby limits reduction in theeffective perpendicular magnetic anisotropy energy.

FIG. 3 is a graph showing signal and noise level variations with respectto recording densities in kFRPI (kilo flux revolutions per inch) for thesignal-layer PMR disk shown in FIG. 1 and the pseudo double-layer PMRdisk with the intermediate soft magnetic layer shown in FIG. 2. In FIG.3, -▪- and -□- represent the signal and noise levels, respectively, ofthe single-layer PMR disk, and -●- and -◯- represent the signal andnoise levels, respectively, of the pseudo double-layer PMR disk.

The pseudo double-layer PMR disk shows a higher signal output than thesingle-layer PMR disk due to retention of the effective perpendicularmagnetic anisotropy energy by the intermediate soft magnetic layer 26that reduces the demagnetization energy by forming a closed magneticcircuit through the perpendicular magnetic recording layer 23. However,the intermediate soft magnetic layer 26 is also likely to cause a randomorientation of neighboring magnetic fields and results in additionalnoise (jitter), so the pseudo double-layer PMR disk has a higher noiselevel than the single-layer PMR disk. Due to increases in both thesignal and noise levels, the pseudo double-layer PMR disk has asignal-to-noise ratio which is too small for high-density recording.Therefore, there is a need to reduce a noise output level originatingfrom the perpendicular magnetic recording layer 23 of the pseudodouble-layer PMR disk to obtain a SNR large enough for high-densityrecording.

Reducing a noise level is also advantageous to the signal-layer PMR diskfor improved SNR. For this reason, there have been continuing efforts toreduce a noise level amplified by the perpendicular magnetic recordinglayer itself in the single-layer and pseudo double-layer PMR disks forimproved SNR.

SUMMARY OF THE INVENTION

To solve the above-described problems, it is an objective of the presentinvention to provide a perpendicular magnetic recording (PMR) disk witha single-layered structure or a pseudo double-layered structureincluding an intermediate soft magnetic layer to reduce thedemagnetization energy of a perpendicular magnetic recording layer, inwhich amplification of a noise level occurring together with signallevel increase is reduced with a stable signal-to-noise ratio (SNR) frommagnetic domains.

To achieve the objective of the present invention, there is provided aperpendicular magnetic recording disk including an underlayer between asubstrate and a perpendicular magnetic recording layer for inducingperpendicular orientation of the perpendicular magnetic recording layer,the perpendicular magnetic recording layer having a thickness in therange where the ratio of perpendicular coercivity Hc to maximumperpendicular coercivity Ho decreases with reduced thickness of theperpendicular magnetic recording layer.

Preferably, the perpendicular magnetic recording disk is applied to apseudo double-layer structure including an intermediate soft magneticlayer between the underlayer and the perpendicular magnetic recordinglayer for forming closed magnetic loops together with the perpendicularmagnetic recording layer.

Preferably, in the range of thickness of the perpendicular magneticrecording layer, the rate of variation of the ratio of perpendicularremanent magnetization to maximum perpendicular remanent magnetizationis greater than that of the ratio of perpendicular coercivity Hc tomaximum perpendicular coercivity Ho.

Preferably, in the range of thickness of the perpendicular magneticrecording layer, a noise level constant of proportionality a expressedas the following formula decreases with reduced thickness of theperpendicular magnetic recording layer:$\alpha = \frac{4\quad\pi\quad{Mr}}{Hc}$where Mr is the perpendicular remanent magnetization and Hc is theperpendicular coercivity.

It is preferable that the perpendicular magnetic recording layer isformed of a CoCr alloy. It is preferable that the perpendicular magneticrecording layer further comprises at least one material selected fromthe group consisting of B, Pt, Ta, V, Nb, Zr, Y, and Mo. It ispreferable that the perpendicular magnetic recording layer has athickness of 20-50 nm.

It is preferable that the intermediate soft magnetic layer is formed ofa NiFe alloy. Preferably, the intermediate soft magnetic layer furthercomprises at least one material selected from the group consisting ofNb, V, Ta, Zr, Hf, Ti, B, Si, and P. Preferably, the intermediate softmagnetic layer has a thickness of 3-30 nm.

It is preferable that the perpendicular magnetic recording disk furthercomprises a protective layer and a lubricant layer sequentially on theperpendicular magnetic recording layer.

The PMR according to the present invention is compatible with aring-type magnetic record head and a magneto-resistive (MR) read head.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objective and advantages of the present invention will becomemore apparent by describing in detail preferred embodiments thereof withreference to the attached drawings in which:

FIG. 1 is a sectional view showing the structure of a single-layerperpendicular magnetic recording (PMR) disk;

FIG. 2 is a sectional view showing the structure of a pseudodouble-layer PMR disk with an intermediate soft magnetic layer;

FIG. 3 is a graph showing signal and noise level variations with respectto recording densities in kFRPI (kilo flux revolutions per inch) for thesingle-layer PMR disk and the pseudo double-layer PMR disk;

FIG. 4 is a graph showing variations in perpendicular coercivity Hc withrespect to variations in thickness of a PMR layer;

FIG. 5 is a graph showing variations in domain diameter and proportionalnoise level constant a with respect to variations in thickness of thePMR layer;

FIG. 6 is a graph showing variations in perpendicular coercivity ratioHc/Ho and the perpendicular remanent magnetization ratio Mr/Mo withrespect to variations in thickness of the PMR layer;

FIG. 7 is a graph showing variations in signal and noise levels withrespect to recording densities in fFRPI for single-layer PMR disksmanufactured in Examples 1 and 2 and Comparative Example 1;

FIG. 8 is a graph showing variations in signal-to-noise ratio (SNR) withrespect to recording densities in kFRPI for the single-layer PMR disksmanufactured in Examples 1 and 2 and Comparative Example 1;

FIG. 9 is a graph showing variations in signal and noise levels withrespect to recording densities in KFRPI for pseudo double-layer PMRdisks manufactured in Examples 3 and 4 and Comparative Example 2;

FIG. 10 is a graph showing variations in SNR with respect to recordingdensities in kFRPI for the pseudo double-layer PMR disks manufactured inExamples 3 and 4 and Comparative Example 2; and

FIG. 11 comparatively shows variations in SNR with respect to recordingdensities in kFRPI for the PMR disks manufactured in ComparativeExamples 1 and 2 and Examples 2 and 4.

DETAILED DESCRIPTION OF THE INVENTION

In increasing the signal-to-noise ratio (SNR) of a single-layer orpseudo double-layer perpendicular magnetic recording (PMR) disk, a noiselevel should be reduced while keeping a signal level of a PMR layerconstant. Noise levels are proportional to a noise level constant αwhich is proportional to the average diameter of reversed magneticdomains formed in the magnetic recording layer, as expressed by formula(2) below: $\begin{matrix}{\alpha = \frac{4\quad\pi\quad{Mr}}{Hc}} & (2)\end{matrix}$where Mr is the perpendicular remanent magnetization and Hc is theperpendicular coercivity. Therefore, there is a need to reduce thediameter of magnetic domains in the magnetic recording layer to reducethe noise level.

The domain diameter in the magnetic recording layer is dependent on thebalance between the magnetostatic energy and domain wall energy. Inparticular, to lower the magnetostatic energy, there is a need to dividedomains in the magnetic recording layer into a number of micro-domainsto form closed magnetic loops. However, the domain wall energy isincreased due to the increased number of micro-domains, therebyincreasing the total energy level of the PMR layer. The sum ofelectrostatic energy and domain wall energy and the domain diameter havethe following relationship:E _(tot) =E _(s) +E _(wall)=1.7Ms ² D+γL/D  (3)where E_(tot) is the total energy of the PMR layer, E_(ms) is theelectrostatic energy of the PMR layer equivalent to 1.7 Ms²D, E_(wall)is the domain wall energy of the PMR layer equivalent to γL/D, Ms is thesaturation magnetization, D is the domain diameter, γ is the domain wallenergy, and L is the thickness of the PMR layer.

To minimize the total energy of the PMR layer, i.e., the sum of theelectrostatic energy and domain wall energy, expressed as formula (3)above, the domain diameter D is determined based on formula (4) below:$\begin{matrix}{D = {\sqrt{\frac{\gamma\quad L}{1.7M_{S}^{2}}}.}} & (4)\end{matrix}$

As is apparent from formula (4) above, the thickness L of the PMR layerin a single-layer or pseudo double-layer PMR disk can be decreased toreduce the domain diameter D in the PMR layer and thereby to lower noiselevels.

In manufacturing a conventional PMR disk, to reduce a noise level of itsPMR layer, the thickness of the PMR layer is determined to be a point atwhich the perpendicular coercivity Hc has a maximum value, therebyresulting in a minimal value of the noise level constant ofproportionality α expressed as formula (2) above. As an example, for aCoCr alloy magnetic recording layer of a conventional PMR disk, theperpendicular coercivity Hc is abruptly decreased at a recording layerthickness no greater than 50 nm, as shown in FIG. 4, which shows theratio of perpendicular coercivity Hc to maximum perpendicular coercivityHo with respect to thickness variations. Thus, the CoCr alloy magneticrecording layer is formed to be thicker than 50 nm in the conventionalPMR disk for noise level control.

However, a high-coercivity magnetic recording layer as thick as 50 nm orgreater is not enough to reduce the noise level constant ofproportionality α, as shown in FIG. 5, for a pseudo double-layer PMRdisk. Also, there occur additional noises (jitter) in the pseudodouble-layer PMR disk due to the use of the intermediate soft magneticlayer, thereby resulting in a high noise level. Accordingly, the SNR ispoor.

FIG. 5 shows the variations in domain diameter with respect tovariations in thickness of a CoCr alloy magnetic recording layer in apseudo double-layer PMR disk. As shown in FIG. 5, decreases in domaindiameter are observed at a magnetic recording layer thickness smallerthan the thickness at which the perpendicular coercivity Hc starts todecrease. Apparently, micro-domains can be formed at a reduced thicknessof the magnetic recording layer. Also, the formation of micro-domains inthe magnetic recording layer can induce a sharp reduction of the noiselevel constant of proportionality a, as shown in FIG. 5.

FIG. 6 is a graph showing the variations in perpendicular coercivityratio (Hc/Ho) and perpendicular remanent magnetization ratio (Mr/Mo)with respect to variations in thickness of the CoCr alloy magneticrecording layer in a pseudo double-layer PMR disk. In FIG. 6, Hcrepresents the perpendicular coercivity, Ho represents the maximumperpendicular coercivity, Mr represents the perpendicular remanentmagnetization, and Mo represents the maximum perpendicular remanentmagnetization.

As shown in FIG. 6, the perpendicular remanent magnetization ratio(Mr/Mo) shows a sharp reduction with respect to variations in thicknessof the CoCr alloy magnetic recording layer, compared to theperpendicular coercivity ratio (Hc/Ho). This sharp reduction in theperpendicular remanent magnetization ratio (Mr/Mo) with reducedthickness of the magnetic recording layer, which is due to the formationof micro-domains, reduces the noise level constant of proportionality α.

In a pseudo double-layer PMR disk according to the present invention,the thickness of a CoCr alloy magnetic recording layer is reduced to athickness at which the perpendicular coercivity decreases, andmicro-domains are formed in the CoCr alloy magnetic recording layer,thereby lowering noise levels with improved SNR. The inventors of thepresent invention also have experimentally found the same effect in asingle-layer PMR disk.

According to a preferred embodiment of the present invention, a pseudodouble-layer PMR disk may be manufactured as follows. An underlayer thatinduces the perpendicular orientation of a PMR layer is formed to athickness of 50-100 nm on a glass or aluminium (Al) alloy substrate byvacuum deposition, an intermediate soft magnetic layer is formed on theunderlayer to a thickness of 3-30 nm, and then the PMR layer is formedthereon to a thickness of 20-50 nm. Next, a protective layer and alubricant layer are sequentially formed on the PMR layer.

A single-layer PMR disk can be manufactured in the same manner as abovefor the pseudo double-layer PMR disk, except that the intermediate softmagnetic layer is not formed.

Suitable materials for the underlayer for promoting the perpendicularorientation of the PMR layer include a titanium (Ti) alloy, anon-magnetic cobalt (Co) alloy, a platinum (Pt) alloy, and a palladium(Pd) alloy, but a Ti alloy is preferred.

In the present invention, the protective layer and the lubricant layercan be formed of any material to an arbitrary thickness within apredetermined range for normal deposition in the field withoutlimitations.

The present invention will be described in greater detail by means ofthe following examples. The following examples are for illustrativepurposes and are not intended to limit the scope of the invention.

EXAMPLE 1

A Ti underlayer was deposited to a thickness of 50 nm on a glasssubstrate having a 650-nm thickness. A CoCr alloy PMR layer was formedon the Ti underlayer to a thickness of 35 nm, a carbon-based layeracting as a protective layer was formed thereon to a thickness of 10 nm,and a lubricant layer was formed thereon to a thickness of 2 nm, therebyresulting in a single-layer PMR disk.

EXAMPLE 2

A single-layer PMR disk was manufactured in the same manner as inExample 1 except that the thickness of the PMR layer was 20 nm.

EXAMPLE 3

A Ti underlayer was deposited to a thickness of 50 nm on a glasssubstrate having a 650-nm thickness. An intermediate NiFe alloy softmagnetic layer was formed thereon to a thickness of 20 nm, and a CoCralloy magnetic recording layer was formed as a PMR layer thereon to athickness of 35 nm. Next, a carbon-based layer acting as a protectivelayer was formed thereon to a thickness of 10 nm, and a lubricant layerwas formed thereon to a thickness of 2 nm, thereby resulting in a pseudodouble-layer PMR disk.

EXAMPLE 4

A pseudo double-layer PMR disk was manufactured in the same manner as inExample 3 except that the thickness of the PMR layer was 20 nm.

Comparative Example 1

A single-layer PMR disk was manufactured in the same manner as inExample 1 except that the thickness of the CoCr alloy magnetic recordinglayer as a PMR layer was 50 nm.

Comparative Example 2

A pseudo double-layer PMR disk was manufactured in the same manner as inExample 3 except that the thickness of the CoCr alloy magnetic recordinglayer as a PMR layer was 50 nm.

FIG. 7 shows variations in signal and noise levels with respect torecording densities in kFRPI for the single-layer PMR disks manufacturedin Examples 1 and 2 and Comparative Example 1. As shown in FIG. 7, thesignal level increases with reduced thickness of the PMR layer due toincrease in perpendicular magnetic field gradient. Also, the noise levelis markedly decreased due to formation of micro-domains.

FIG. 8 shows variations in SNR with respect to recording densities inkFRPI for the single-layer PMR disks manufactured in Examples 1 and 2and Comparative Example 1. As shown in FIG. 8, the SNR improves withreduced thickness of the PMR layer, due to the increase in signal leveland decrease in noise level as described in associated with FIG. 7.

FIG. 9 shows variations in signal and noise levels with respect torecording densities in kFRPI for the pseudo double-layer PMR disksmanufactured in Examples 3 and 4 and Comparative Example 2. As shown inFIG. 9, the signal level increases with reduced thickness of the PMRlayer, due to increase in perpendicular magnetic field gradient and dueto the reduction of demagnetisation by formation of the soft magneticlayer. Also, the noise level is markedly decreased due to formation ofmicro-domains.

FIG. 10 shows the variations in SNR with respect to recording densitiesin kFRPI for the pseudo double-layer PMR disks manufactured in Examples3 and 4 and Comparative Example 2. As shown in FIG. 10, the SNR improveswith reduced thickness of the PMR layer, due to the increase in signallevel and decrease in noise level as described in associated with FIG.9.

FIG. 11 comparatively shows the variations in SNR with respect torecording densities in kFRPI for the conventional single-layer andpseudo double layer PMR disks having a 50-nm-thick CoCr alloy magneticrecording layer and for the single-layer and pseudo layer PMR diskshaving a 20-nm-thick CoCr alloy magnetic recording layer according tothe present invention. A great improvement in SNR is observed for thepseudo double layer PMR disk according to the present invention having amagnetic recording layer as thin as 20 nm with micro-domains.

As described above, in a PMR disk according to the present invention,micro-domains are formed in a magnetic recording layer based on therelation between the magnetostatic energy and domain wall energy of thePMR layer. The PMR layer with micro-domains is applied to a single-layerPMR disk or a pseudo double-layer PMR disk with closed magnetic loops,thereby reducing noise level and improving SNR.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1-11. (canceled)
 12. A method of forming a perpendicular magneticrecording media, comprising: forming an underlayer on a substrata;forming an intermediate soft magnetic layer on the underlayer; andforming a perpendicular magnetic recording layer on the intermediatesoft magnetic layer, wherein the underlayer is adapted to induce aperpendicular orientation in the perpendicular magnetic recording layer,wherein the intermediate soft magnetic layer is adapted to form closedmagnetic loops together with the perpendicular magnetic recording layer,and wherein the perpendicular magnetic recording layer is formed to athickness in the range where the ratio of perpendicular coercivity Hc tomaximum perpendicular coercivity Ho decreases with reduced thickness ofthe perpendicular magnetic recording layer.
 13. The method of forming aperpendicular magnetic recording media of claim 12, wherein, in therange of thickness of the perpendicular magnetic recording layer, therate of variation of the ratio of perpendicular remanent magnetizationof maximum perpendicular remanent magnetization is greater than of theratio of perpendicular coercivity Hc to maximum perpendicular coercivityHo.
 14. The method of forming perpendicular magnetic recording media ofclaim 12, wherein, in the range of thickness of the perpendicularmagnetic recording layer, a noise level constant of proportionality αexpressed as the following formula decreases with reduced thickness ofthe perpendicular magnetic recording layer:$\alpha = \frac{4\quad\pi\quad{Mr}}{Hc}$ where Mr is the perpendicularremanent magnetization and Hc is the perpendicular coercivity.
 15. Themethod of forming a perpendicular magnetic recording media of claim 12,wherein the perpendicular magnetic recording layer is formed of a CoCralloy.
 16. The method of forming a perpendicular magnetic recordingmedia of claim 15, wherein the perpendicular magnetic recording layerfurther comprises at least one material selected from the groupconsisting of B, Pt, Ta, V, Nb, Zr, Y, and Mo.
 17. A method of forming aperpendicular magnetic recording media, comprising: forming anunderlayer on a substrate; forming an intermediate soft magnetic layeron the underlayer; and forming a perpendicular magnetic recording layeron the intermediate soft magnetic layer, wherein the underlayer isadapted to induce a perpendicular orientation in the perpendicularmagnetic recording layer, wherein the intermediate soft magnetic layeris adapted to form closed magnetic loops together with the perpendicularmagnetic recording layer, wherein the perpendicular magnetic recordinglayer is formed to a thickness in the range where the ratio ofperpendicular coercivity Hc to maximum perpendicular coercivity Hodecreases with reduced thickness of the perpendicular magnetic recordinglayer, and wherein the perpendicular magnetic recording layer is formedof a CoCr alloy including at least one material selected from the groupconsisting of B, Pt, Ta, V, Nb, Zr, Y, and Mo and has a thickness of20-50 nm.
 18. The method of forming a perpendicular magnetic recordingmedia of claim 12, wherein the intermediate soft magnetic layer isformed of a NiFe alloy.
 19. The method of forming a perpendicularmagnetic recording media of claim 18, wherein the intermediate softmagnetic layer further comprises at least one material selected from thegroup consisting of Nb, V, Ta, Zr, Hf, Ti, B, Si, and P.
 20. The methodof forming a perpendicular magnetic recording media of claim 19, whereinthe intermediate soft magnetic layer has a thickness of 3-30 nm.
 21. Themethod of forming a perpendicular magnetic recording media of claim 12,further comprising forming a protecting layer and a lubricant layersequentially on the perpendicular magnetic recording layer.